1. Field of the Invention
The present invention relates to a substrate, using a nitride type compound semiconductor such as gallium nitride (GaN), for light-emitting devices such as light-emitting diodes and semiconductor lasers, and electronic devices such as field-effect transistors; and a method of making the same.
2. Related Background Art
In light-emitting devices using nitride type compound semiconductors, and the like, stable sapphire substrates have conventionally been used.
Since sapphire has no cleavage surfaces, however, it has been problematic in that a reflecting surface cannot be made by cleavage when a sapphire substrate is employed for a semiconductor laser.
There is also a problem that, when sapphire is employed as a substrate material for a light-emitting device or the like, there occurs a lattice mismatch or difference in coefficient of thermal expansion between the sapphire substrate and an epitaxial layer grown thereon, whereby crystal defects such as dislocation often occur in the epitaxial layer.
As a technique developed in order to overcome such a problem in the case where sapphire is employed as a substrate for a light-emitting device or the like, there is a method of making a semiconductor light-emitting device disclosed in Japanese Patent Application Laid-Open No. HEI 8-116090. This method of making a semiconductor light-emitting device comprises the steps of growing a gallium nitride type compound semiconductor layer on a semiconductor single crystal substrate such as an gallium arsenide (GaAs) substrate; eliminating the semiconductor single crystal substrate (GaAs substrate) thereafter; and using the remaining gallium nitride compound semiconductor layer as a new substrate and epitaxially growing a gallium nitride type compound semiconductor single crystal layer as an active layer thereon, thereby making the semiconductor light-emitting device.
According to the technique of Japanese Patent Application Laid-Open No. HEI 8-116090, the lattice constant and coefficient of thermal expansion of the gallium nitride compound semiconductor layer are very close to those of the gallium nitride compound semiconductor single crystal layer (epitaxial layer) grown thereon, so that lattice defects due to dislocation or the like are harder to occur in the semiconductor single crystal layer (epitaxial layer). Also, since the substrate and the active layer grown thereon are made of the same gallium nitride type compound semiconductor layer, the same kind of crystals align with each other, whereby they can easily be cleaved. Consequently, reflecting mirrors for semiconductor lasers and the like can easily be produced.